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or mischief, as the case may be. On very dry days in Denver the amount of crime among adults and of misconduct among school children increases largely. The nerves seem to become unstrung by reason of the high state of electric or magnetic tension, by the dryness and the wind. When the wind dies down and the air becomes moist, the nerves return to their normal condition, but the system has been through an experience which reduces the power to control emotional impulses. We find that people in extremely hot, dry countries, like Persia and Chinese Turkestan, are highly emotional and seriously lacking in self-control.

Each kind of climate and the geographical characteristics of every inhabited region exert more or less influence upon the industrial life and the social organization of the people. If the plain is waterless in summer and the plateau deeply buried in snow in the winter, the animals must migrate. Man finds the region too dry in one part and too cold in another part for agriculture. Therefore he must live upon animals, either as a hunter, or, after he has partially domesticated some species of animal, as a shepherd. This leads to a nomadic life, which in turn induces habits of cleanliness in eating, traveling, sleeping, working, and resting. Such habits becoming mass phenomena or usages of the group, develop moral standards of abstemiousness, hardihood under physical difficulties, laziness, hospitality, and the like. Thus the physical features mold the people. Geographical environment has an important influence upon the forms of invention. Protection against exposure is attained in accordance with the available materials; for example, the snow house of the Eskimo, the bark wigwam of the Indian, and the cave dwelling of the tribes of the

desert. The complex bows of the Eskimo appear to be due to the lack of any long elastic material for bowstaves, and various devices have been invented for securing elasticity of the bow where elastic wood is difficult to obtain. Tribes without permanent habitation resort to skin receptacles and baskets as substitutes for pottery.32

During the thousands of years before history was written primitive men were subjected to the varying climatic influences which we have described. These climatic influences were conditions to which primitive men had to adjust and adapt themselves as best they might. A great climatic change which caused the desiccation of a large and highly populated area killed off its human inhabitants by thousands. Those whose constitutions were plastic enough to withstand the change and make the necessary adaptations survived; others perished or migrated to more favorable territory. In the course of migrations, these early peoples not possessing our knowledge of means of transportation and communication, were subordinated to the natural barriers or means of travel such as mountain masses and valleys. The surface of the earth has determined the movements of populations and the migrations of races from those areas which climatic changes have made uninhabitable.

Valleys offer channels for the easy movement of hu manity. They are grooves which have time and again determined the destination of aimless, unplanned migrations. The passing of peoples follows these naturemade highways. "The maritime plain of Palestine has been an established route of commerce and war from the time of Sennacherib to Napoleon." Up the Danube valley

32 Boas, op. cit., p. 160.

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From Coman, "The Industrial History of the United States."

FIGURE 53. Topography and Migration, Roads and Trails into the Western

Territory.

have pressed long series of barbarian invaders from Attila the Hun to the Turkish besiegers of Vienna in 1683. The river is a great natural highway to which every neighboring state desires access. In America, the Mohawk depression through the northern Appalachians diverts a significant amount of Canada's trade from the Great Lakes to the Hudson.33 Formerly it enabled the Dutch traders at New Amsterdam to tap the fur trade of Canada's forests, and later, after the construction of the Erie canal, enabled New York to defy the competition of Boston, Philadelphia and Baltimore, in providing the easiest outlet for the commerce of the rich Ohio valley. The Cumberland Gap was the natural avenue to the West from Virginia and the Carolinas. Buffalo, Indian and pioneer have successively followed this route.34 Natural conditions have fixed channels in which the stream of humanity most easily moves.35 The direction of mountain ranges determines within certain limits. the destination of migration, and this tends to keep succeeding waves to the old channels. These lines of least resistance are first sought out and only when they are blocked or preëmpted do the invaders turn to more difficult paths.

The long and narrow valley of the Nile, with its fertile hem of flood-plain on either bank and the protecting barrier of the great desert beyond, furnished conditions favorable to the development of a great civilization. Here was a rich soil kept in splendid condition by the annual flood period which replenished the vital mineral and organic elements withdrawn by the crops, so that

33 Semple, op. cit., p. 5.

34 Semple, E. C.-American History and Its Geographic Conditions, p. 68. 35 See figure 53.

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